What is colon:message `:message` in form - html

Question

I want to know more about how :message works. I see in the official docs how to use it but I don't understand where the value is set and how it becomes a unique variable for each input. I can't it find it in the API either.
<div class="form-group {{ $errors->has('first_name') ? 'has-error' : '' }}">
<label for="first_name">First Name</label>
<input type="text" class="form-control" id="first_name">
{{ $errors->first('first_name', '<p class="help-block">:message</p>') }}
</div>
where can I find more information about :message. what does the : signify and so on? I can't google it. So I hope someone can direct me to a reference.

Answer 1

What you pass in as second argument to first() is the format of the error message. As mentioned in the comments, the :message part gets replaced by the actual error message.
It all happens inside the MessageBag class
You call first(). Inside the function $this->get() gets called
$messages = is_null($key) ? $this->all($format) : $this->get($key, $format);
Inside get it will check if there's a message for the passed key and if positive $this->transform() will be called
if (array_key_exists($key, $this->messages))
{
return $this->transform($this->messages[$key], $format, $key);
}
Now it finally gets interesting. Within transform, The keywords :message and :key will be replaced with the actual message and the actual key ($format is the string you passed in at the beginning '<p class="help-block">:message</p>')
$replace = array(':message', ':key');
$message = str_replace($replace, array($message, $messageKey), $format);